250 Dr Richardson's Observations on Solar Radiation, 

 TABLE VII. 



Exhibiting the Monthly Mean difference of Temperature in the Shade 

 at Sunrise, from the Mean Maximum at 1 or 2 P.M. 



Note. — The mean temperature in the third column is for the most part 

 that at 1 p.m. ; but when the temperature at 2 was greater, that is given. 



Remarks on the Preceding Paper. By Professor Forbes. 



Dr Richardson has, I think, fairly deduced from his obser- 

 vations, confessedly imperfect as they are, that his photo- 

 metric apparatus was more affected by sunlight in March and 

 April than during the summer months. Whether this be due 

 to the greater intensity of the solar rays in spring, as he sup- 

 poses, may perhaps be considered as not so fully proved. The 

 principle of measuringthe intensity of solar radiation by ablack- 

 ened thermometer, is due to Lambert, and was ingeniously 

 and elegantly applied by Leslie. Such instruments, carefully 

 sheltered by glass (as in Dr Richardson's later experiments), 

 are certainly capable of yielding valuable results, although 

 we have not yet learned to interpret them aright. Their in- 

 dications are, however, very different from the sensible effects 

 of the sun's action on the animal frame for instance, and from 

 the more direct measure of it which is obtained by means of 

 Herschel's Actinometer.* Any measures sufficiently often re- 



* It is a curious fact, which has only come lately to my knowledge, that 

 in Sir J. Leslie's earliest paper, read to the Royal Society in 1793, but first 

 published twenty-six years later in Thomson's Annals of Philosophy, vol. 

 xiv., he has laid down with perfect clearness the principle of the actinome- 

 ter, which he has described as the only true measure, and which yet ho 

 wholly overlooked in the final construction of his Photometer, which mea- 

 sures the statical maximum of temperature which a blackened ball is ca- 

 pable of assuming, instead of the momentary increment of heat wliich it 

 receives. His words are : " The initial change on the thermometer is in 



